Process and equipment for the treatment of a material by means of an arc discharge plasma

ABSTRACT

A process is described wherein a material, such as particulate metal oxides, is treated with an arc discharge plasma under vacuum in the presence of a magnetic field running axially between the arc electrodes, wherein the plasma rotates symmetrically about a middle magnetic field, under conditions such that in a region the product of the electron gyration frequency Omega and time Tau , within which the average electron transmits its impulse to the plasma ions, is Omega Tau &gt; 1 at about the middle of the magnetic field, and treated material, i.e., reduced metal, is recovered radially outside the region. Apparatus is described, for carrying out the process, including a discharge vessel, two spaced annular electrodes arranged symmetrically to the axis of the vessel, a magnetic coil enclosing coaxially the space between the electrodes, and means to introduce material axially through one electrode into the space.

llnited States Patent 1 1 1111 3,852,061 Wulif 1 Dec. 3, 1974 PROCESS AND EQUIPMENT FOR THE 3,771,585 11/1973 Ulrich 75/10 TREATMENT OF A MATERIAL BY MEANS OF AN ARC DISCHARGE PLASMA lnventor: Heinrich Wulft, Munich, Germany Assigneet Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V.,

Gottingen, Germany Filed: Nov. 20, 1972 Appl. No.: 307,838

Foreign Application Priority Data Nov. 20, 1971 Germany 2157606 US. Cl. 75/10 R, 13/9, 75/.5 BB Int. Cl. C22d 7/00, H05b 7/18 Field of Search 75/10, .5 BB; 204/164;

References Cited UNITED STATES PATENTS 10/1961 Candidus 7 5/10 3/1963 Hanks 75/65 EB 2/1967 Death 204/104 12/1970 Hutchinson 75/10 12/1971 Bruning Primary Examiner-L. Dewayne Rutledge Assistant ExaminerPeter D. Rosenberg Attorney, Agent, or FirmRusse1l & Nields [5 7] ABSTRACT A process is described wherein a material, such as particulate metal oxides, is treated with an arc discharge plasma under vacuum in the presence of a magnetic field running axially between the arc electrodes, wherein the plasma rotates symmetrically about a middle magnetic field, under conditions such that in a region the product of the electron gyration frequency Q and time 7, within which the average electron trans-' mits its impulse to the plasma ions, is 07 1 at about the middle of the magnetic field, and treated material, i.e., reduced metal, is recovered radially outside the region.

Apparatus is described, for carrying out the process, including a discharge vessel, two spaced annular electrodes arranged symmetrically to the axis of the vessel, a magnetic coil enclosing coaxially the space between the electrodes, and means to introduce material axially through one electrode into the space.

,1 Ewing Figure PROCESS AND EQUIPMENT FOR THE TREATMENT OF A MATERIAL BY MEANS OF AN ARC DISCHARGE PLASMA SUMMARY OF THE INVENTION The present invention relates to a process and equipment for the treatment of a material, such as the reduction of metal oxides, by means of an arc discharge plasma burning in a discharge space.

BACKGROUND OF THE DISCLOSURE Inductive plasma burners for the heating of finegrained material are known which contain a cylindric discharge vessel into whose front side streams a mixture of a gas and the fine-grained material in axial direction. The stabilization of the discharge plasma is achieved through a tangentially introduced auxiliary current of gas. Such plasma burners are especially used for the melting of powdery or granular materials of high fusing temperatures, such as fireproof oxides or carbides as well as for flame spraying (See German patent publication No. 1,286,241).

Further, there are known plasma burners and equipment for the heat treatment of materials whereby the plasma is produced through an arc discharge burning between two electrodes. Such equipment has an advantage over high-frequency plasma burners, in that the efficiency is higher and the apparatus is smaller and less costly.

US. Pat. No. 3,051,639 discloses equipment for hydrocarbon reactions, which employs an arc discharge I between a rod-shaped central electrode and an annular' electrode arranged at an axial distance from its point. No magnetic field is, however, provided.

US. Pat. No. 2,944,140 discloses plasma burners in which the plasma is produced through an arc discharge between a rod-shaped or platelike first electrode, and an annular electrode arranged at a distance from this. In the space between the two electrodes, enclosed by a cylindrical wall, a current of gas is tangentially introduced. For the stabilization of the plasma jet issuing through the opening of the annular electrode, there is provided a magnetic nozzle which is formed through a magnetic coil that may be arranged either between the electrodes or on the side of the annular electrode that is facing away from the platelike electrode. In the latter case the cross-section of the coil, consisting of a tube, through which flows a cooling agent, may at first somewhat diminish and then again gradually enlarge with increasing distance from the annular electrode.

In a plasma burner disclosed in US. Pat. No. 2,945,l 19 which likewise works with a magnetic nozzle", the plasma jet produced through an arc discharge between a platelike electrode and an annular electrode passes through a quartz tube fixed to the central opening of the annular electrode-into wall of which two spaced annular electrodes are inserted. Theannular electrodes are connected with a direct-current supply for producing an auxiliary discharge. Between the annular electrodes is arranged a cylindric magnetic coil, connected with a direct-current source, enclosing the quartz tube, whose length is considerably smaller than the axial distance from the annular electrode.

There is further known from the German publication 1,932,703 equipment for the heat treatment of materials through an arc plasma, whereby the arc discharge burns between a tapered conical anode and a plasma jet serving as cathode, axially projecting into the anode, which is produced through a linear plasmatron. The anode is enclosed by a magnetic coil, or is itself developed as a magnetic coil in order to produce a substantially axial magnetic field which exerts with the radial electric field between the plasma jet and the anode an azimuthal force on the arc in order to let it rapidly r0 tate, so that the temperature in the reaction area will be more uniformly distributed. The pressure in the reaction space enclosed by the anode may be equal to atmospheric pressure, smaller than it, or greater than it. On reduced pressure the discharge may be diffuse, whereas on higher pressures there develops an arc discharge. The material that is to be treated is introduced through a laterally obliquely inserted tube approximately between the axis and the circumference of the anode into the upper part of the space enclosed by it. Underneath the anode is provided a freezing device.

Finally, it is known from scientific publications (Physics Letter, 24A, No. 6, Mar. 13, 1967, p.324/325; Z. Naturforsch. 230, 251-263, 1968 and 24a, 1473-1491, 1969) that between two annular electrodes, arranged at a distance from one another, between which lies a relatively strong magnetic field, there can be produced a stable arc discharge that has some unusual properties. The conditions for the existence of such a discharge are, however, relativelycritical, e.g., it is a prerequisite for the existence of such a discharge that the electron density lies in the range between about 5 X 10 and 3 X l0 cm It is a disadvantage of the known processes and equipment for the treatment of materials by means of a plasma, produced through an electric discharge between two electrodes, that contamination of the treated material with electrode material is practically unavoidable. Moreover, there exists practically no zone of uniform temperature and density; on the contrary mostly the attempt is made to approximate such a zone through a rapidly rotating discharge channel.

The present invention has as a major objective to obtain a process and equipment for the treatment of material by means of an arc discharge plasma, whereby a contamination of the material with electrode material is prevented and a stable discharge is assured, in the presence of a zone of uniform, very high temperature.

BRIEF DESCRIPTION OF THE INVENTION The invention includes a process characterized by a combination of the following conditions:

a. the average pressure in the discharge space containing the electrodes is kept below atmospheric pressure;

b. a magnetic field is produced which runs substantially parallel to the axis forming the connection line between the electrodes, and which has in at least one part of a region of such a high value that the product 01 of the electron gyration frequency in the plasma Q and the time -r within which on an average an electron c. the material to be treated is brought into a part of the region close to the central 'magnetic field line in which is fulfilled the condition of 01- 1; and

d. the treated material is recovered from the space lying radially outside of this region.

. Under the above conditions, there occurs a great radial pressure gradient which results in high pressures in the axial region. Thereby contamination emanating from the electrode is kept away from the part of the discharge space which lies between the electrodes. On the other hand, surprisingly, the material introduced into the space between the electrodes, preferably at an axial distance from them in the proximity of the axis, is actually not transported in axial direction, but it permeates the rotating discharge in radial direction, so that the treated material is obtained in pure state from the part of the discharge space which lies outside of the plasma, e.g. after it has been centrifuged onto the inside wall of the discharge vessel. Unexpectedly, the discharge is also not so strongly disturbed by the foreign or untreated material as to cause instabilities. We have found that the relatively high electron temperature in the uninfluenced discharge, to be sure, decreases with the introduction of foreign material, but that the electron density which is required for the stability and existence of the discharge is hardly influenced by the introduction of the material.

BRIEF DESCRIPTION OF THE DRAWING The attached FIGURE shows in cross-sectional elevation the preferred apparatus of the invention, an arc discharge plasma vessel for treatment of particulate material in accordance with the present process.

DETAILED DESCRIPTION OF THE INVENTION A preferred application of the present process is for the reduction of metal oxides, such as tantalum oxide, titanium oxide, aluminum oxide for recovery of the pure metal. Preferably, a relatively coarse-grained starting material (e.g., with particle size up to 100 um and more, preferably between 20 and 80 um, such as 50 pm) is used. The particle size is preferably also relatively uniform.

Preferred apparatus for executing the process of the invention contains a discharge vessel-connected with a vacuum pump system,including: an annular first electrode; a second electrode, substantially symmetrical to the vessel axis and arranged at a distance from the first electrode, a magnetic coil coaxially enclosing the space between the electrodes, which is capable of producing in the range of the vessel axis a magnetic field of at least kg (kilo-Gauss); and means for introducing material to be treated through the annular, first electrode into the region of the discharge space between the two electrodes, near the vessel axis preferably at distance axially from the electrodes. Alternative embodiments will also be described hereafter.

The apparatus schematically presented in the drawing including a substantially cylindric vacuum vessel 10 composed, for example, of quartz, ceramic, or any other non-magnetic material. The vacuum vessel 10 is closed at one end, and connected at theother end through a connecting duct 12 with a vacuum system (not shown) which permits evacuation of the vacuum vessel 10, or alternatively to fill it with a desired gas under a specified pressure, to drain the accumulating inner and outer rim of the electrodes 16 and 18 project each approximately tapered conical walls 20a, 20b, and 22a, 2219, respectively, of insulating material, e.g., quartz or ceramic, which define a channel of annular cross section. Preferably, each inner wall 20b or 22b, axially projects farther toward the center of the discharge vessel than its respective outer wall, 20a or 22a.

In the space enclosed by the wall 20b is provided means for the dosed introduction of particulate material 24, e.g., finely-grained or powdery, that is to be treated. The feeding is preferably achieved axially symmetrically with respect to axis 14 of the vessel 10 in order to assure uniform influence on the material through the discharge plasma. The means for feeding the material in the illustrated embodiment may be needle valve 26, adjustable through an electro-magnetic or other control means 28, and permits measured introduction of the material 24 into a region near the axis of the vacuum vessel 10. Since wall 20b projects axially from the electrode 16 into the discharge space enclosed of the vacuum vessel 10, the material to be treated may be introduced into the plasma at considerable axial interval from the electrodes 16 and 18. This, together with the special kind of discharge, help to prevent contamination of the material with electrode material, as will be further explained.

The central part of the vacuum vessel 10 lying between the electrodes 16 and 18 is enclosed by a cylindric magnetic coil 30, which permits the production of a relatively strong magnetic field B. The intensity of the magnetic field should be at least so great that the product Or, of the gyration frequency 0 of the free electrons in the plasma and the time 1 within which on an average an electron transmits its impulse to the ions of the plasma, is greater than 1 at the pressure condition existing in the discharge space, In practice the intensity of the magnetic field will be at least 10 k6, preferably at least 20 k0; good results were obtained at field intensities between 30 and 60 k6.

The magnetic coil terminates preferably bilaterally at an axial distance from the electrodes, so that the magnetic field lines diverge in the region of the electrodes as indicated through dash-lines. The walls 20a, 20b, and 22a, 22b are preferably so shaped (thus tapered conically and similar to a rotated hyperbola), that they follow substantially the course of the magnetic field lines, and that the charge carriers in the discharge are therefore compelled onto paths which run parallel to the electrode walls.

The electrodes 16 and 18 are provided with corresponding terminals 32 and 34, respectively, which are connected with a current source (not shown), preferably a direct-current source that is capable to supply sufficient current for the discharge. The magnetic coil 30 in operation is connected with an energy supply (not shown). The magnetic coil 30 may be'a superconductive coil so that it will not require any energy supply in continuous service.

In the operation of the illustrated apparatus the material to be treated 24, e.g., finely granular or powdery tantalum oxide (or titanium oxide or the like) which is to be reduced, is placed within the space enclosed by the wall 20b. The vacuum vessel is then evacuated through the pump duct 12, and subsequently filled with a desired gas (e.g., hydrogen, air or an inert gas) under an inflation pressure between about 1 and torr/preferably between about 3 and 5 torr (measured at room temperature). With the magnetic field B cut in, there is then ignited between the electrodes 16 and 18 an arc discharge by means of a high-tension impulse. During arc discharge the needle valve 26 is opened and material 24 is fed in measured doses into a region of the discharge near the vessel axis. From the axial region the material is driven radially outward through the hot, current-carrying plasma tube which arises between the electrodes due to the discharge. There results an extremely intense and a uniform reciprocal action with the plasma, and the material is centrifuged against the outside wall of the vacuum vessel 10. The treated material, e.g., reduced metallic tantalum, is thereby accumulated at the inside wall of the vacuum vessel 10, as shown at 36.

In one specific embodiment of the invention the magnetic coil 30 had a length of about 30 cm; the magnetic field B produced through the coil 30 had a field intensity of about 50 k0 and between the electrodes 16 and 18 burned a tubular, stable arc discharge with a current of about 2 kA and an approximate arc-drop-voltage of about 300 V. The distance of the electrodes was about 60 cm, the average diameter of the electrodes was about 6 cm. The work was done in pulsed operation, the duration of the impulses was of the order of milliseconds.

When tantalum oxide or titanium oxide, respectively, was used as the material to be treated, the material settled at the wall of the vacuum vessel in the form of metallic tantalum or titanium, respectively, of high purity. The effect of the reduction in relation to the applied electric energy is very high, so that the production cost of produced material is very much cheaper than that of comparable, commercial processes. Good yields were obtained with finely granular starting material of a particle size of about 50 um.

The described process and apparatus have particular only in the upper part. In principle, it is also possible to application in the preparation of chemical elements from their compounds, particularly for the recovery of metals. Those metals which cannot be produced from their ores through reduction with carbon, therefore are of particular significance, such as titanium, zirconium, vanadium, tantalum, and aluminum. The present process and apparatus can also be used for the preparation of chemical compounds, especially when these can be produced only through strongly endothermic reactions. Moreover, various materials which exist in fusible form can be treated; also liquids of low vapor pressure; or vapors, gases, and mixtures or dispersions of such materials.

The concept annular electrode is meant herein to comprise also electrode forms which topologically are equivalent to a circular ring, thus e.g., perforated electrodes which substantially have the form of a disc, a rectangle, an ellipse, a triangle, and so on.

With the aid of the drawing, we have described a preferred embodiment of the present invention. But from this preferred embodiment there can be made modifications. In some respects, however, certain disadvantages may be encountered. Instead of the annular electrode 18, there may be used a coaxial rod-shaped electrode. The developing plasma discharge is then hollow make both electrodes compact, e.g., rod-shaped, and even to arrange them unsymmetrically with respect to the cylindric magnetic coil 30. The electrodes may also be arranged at a relatively great distance from the magnetic coil. In this case the essential conditions for the present process of QT 1 will only be fulfilled in a part of the range lying between the electrodes, and will develop only there the'plasma discharge which rotates around its axis in the form of a column. Since the plasma discharge is then not hollow, it is not possible to introduce the material to be treated on a burning discharge into the region of the axis of rotation of the plasma discharge, but it becomes first necessary to bring a certain quantity of the material into the region of the axis of rotation, e.g., letting it drop and then only ignite the discharge around the free-falling material. This requires necessarily a relatively complicated control of the course of the process, and the magnetic field is not utilized to its maximum, which fact may lead to quite substantial decreases in efficiency, particularly on account of the high field intensities. The circumstances are more simple if the upper electrode-as shown in the drawingis annular and coaxial to the magnetic field, but arranged so far from this that the conduct of the arc current through the magnetic field is not yet incurred in the proximity of the electrode, and that at the electrode is then initiated a discharge channel, which passes over into discharge form used for treating the material only in the'region in which the condition of QT 1 is fulfilled. Since in this case the desired discharge is not hollow, the discharge is preferably only ignited when the material is already in the range on the axis around which then is formed the ignited, desired discharge.

I claim: a

l. A process for the treatment of material by means of an arc discharge plasma in a region of a discharge vessel between two axially spaced electrodes, comprising the following steps:

a. maintaining during the process the average pressure in said region at less than one atmosphere;

b. producing a magnetic field running substantially parallel to the axis forming the connection line between said electrodes, and which has in at least one part of said region such a high value that the product Q7 of the electron gyration frequency 0 in the plasma and the time 1 within which on an average electron transmits its impulse to the ions of the plasma is greater than 1, and that the arc discharge plasma in said region as a whole in itself rotates around a middle magnetic field line to which the plasma is substantially symmetrical;

c. bringing said material into a part of said region close to the mid-point of said magnetic field under said condition of Qr 1, whereby a reciprocal action with the plasma occurs, and said material is centrifuged radially outward; and

d recovering treated material from the space lying radially outside of said region.

2. The process of claim I, wherein:

a. the discharge is produced between an annular first electrode and a second electrode arranged substantially symmetrical to the axis of said annular electrode and spaced at an axial distance therefrom.

b. said magnetic field is substantially symmetrical to the axis of said annular electrode;

4. The process of claim 1, wherein a neutral gas is within said discharge vessel at a pressure of about 2.5

torr, before ignition of the discharge.

5.The process of claim 1, wherein said magnetic field and discharge are produced in pulsed operation.

6. The process of claim 1, wherein said material is finely-grained having substantially uniform size, in the region between 20 and 80 #m. 

1. A PROCESS FOR THE TREATMENT OF MATERIAL BY MEANS OF AN ARC DISCHARGE PLASMA IN A REGION OF A DISCHARGE VESSEL BETWEEN TWO AXIALLY SPACED ELECTRODES, COMPRISING THE FOLLOWING STEPS: A. MAINTAINING DURING THE PROCESS THE AVERAGE PRESSURE IN SAID REGION AT LESS THAN ONE ATMOSPHERE; B. PRODUCING A MAGNETIC FIELD RUNNING SUBSTANTIALLY PARALLEL TO THE AXIS FORMING THE CONNECTION LINE BETWEEN SAID ELECTRODES, AND WHICH HAS IN AT LEAST ONE PART OF SAID REGION SUCH A HIGH VALUE THAT THE PRODUCT && OF THE ELECTRON GYRATION FREQUENCY && IN THE PLASMA AND THE TIME & WITHIN WHICH ON AN AVERAGE ELECTRON TRANSMITS ITS IMPULSE TO THE IONS OF THE PLASMA IS GREATER THAN 1, AND THAT THE ARC DISCHARGE PLASMA IN SAID REGION AS A WHOLE IN ITSELF ROTATES AROUND A MIDDLE MAGNETIC FIELD LINE TO WHICH THE PLASMA IS SUBSTANTIALLY SYMMETRICAL; C. BRINGING SAID MATERIAL INTO A PART OF SAID REGION CLOSE TO THE MID-POINT OF SAID MAGNETIC FIELD UNDER SAID CONDITION OF &&> 1, WHEREBY A RECIPROCAL ACTION WITH THE PLASMA OCCURS, AND SAID MATERIAL IS CENTRIFUGED RADICALLY OUTWARD; AND D. RECOVERING TREATED MATERIAL FROM THE SPACE LYING RADIALLY OUTSIDE OF SAID REGION.
 2. The process of claim 1, wherein: a. the discharge is produced between an annular first electrode and a second electrode arranged substantially symmetrical to the axis of said annular electrode and spaced at an axial distance therefrom. b. said magnetic field is substantially symmetrical to the axis of said annular electrode; c. said material is introduced through the annular electrode through into said region of the discharge space near said axis, within a hollow plasma tube produced in operation; and d. said treated material is obtained radially outside said plasma tube, terminating axially between said electrodes.
 3. The process of claim 1, wherein said magnetic field has a field intensity of at least 10 kG (kilo-Gauss).
 4. The process of claim 1, wherein a neutral gas is within said discharge vessel at a pressure of about 2.5 torr, before ignition of the discharge.
 5. The process of claim 1, wherein said magnetic field and discharge are produced in pulsed operation.
 6. The process of claim 1, wherein said material is finely-grained having substantially uniform size, in the region between 20 and 80 Mu m. 